Selling and shipping is just like any other place on-line. If you have a PayPal account - that works well. The only catch is that the forum has no legal attachment or otherwise to deals agreed upon. Meaning, if you get burned or burn someone, the forum offers no protections other than perhaps banning someone.
Once you establish selling, trading or whatever with people, the merit/honor code builds. This means we can trust the seller.
The method of payment is up to you, i.e. PayPal, money order, check, cash, trades, etc. Shipping is up to you too, i.e. USPS, Fed Ex, UPS, currier pigeon (for those really smallparts), etc.
"Well, I thought it was a rabbit but it turned out to be Bear Grylls in a rabbit hide."